A plasma processing apparatus is used for various processing such as etching, ashing, plasma deposition and the like for a substrate to be processed, e.g., a semiconductor wafer, a flat panel display (FPD) substrate, or the like. As for the plasma processing apparatus, there is known, e.g., an apparatus in which a planar spiral coil having both ends grounded is provided above a dielectric member and a high frequency power supply is connected to a portion other than the both ends (see, e.g., Patent Document 1). Further, there is formed a standing wave by applying a high frequency power from the high frequency power supply to the spiral coil to make the spiral coil resonate at ½ wavelength (or ¼ wavelength) of the high frequency. Accordingly, an induction field is generated below the dielectric member, and a plasma of a processing gas is excited.
(Patent Document 1)
Japanese Patent Application Publication No. H7-296992 and corresponding U.S. Pat. No. 5,241,245
(Patent Document 2)
Japanese Patent Application Publication No. 2007-142444 and corresponding U.S. Pat. No. 5,965,034
Meanwhile, along with the recent trends toward miniaturized and multilayered semiconductor devices, it is required to perform processing that causes less damage. For example, when the processing is performed by using radicals, it is required to facilitate reaction caused by the radicals while minimizing ion damage. Namely, various studies are being conducted to avoid damage caused by excessive ions, such as mixture of materials forming layers of a wafer, destruction of an oxide film, intrusion of contaminants, change in the characteristics or the like. Further, ion impact that causes a low selectivity needs to be avoided during an etching process or the like that requires a selectivity with high accuracy. Besides, it is known that the ion damage can be effectively suppressed by exciting a plasma having a potential set as low as possible.
However, when both ends of the spiral coil are grounded as in the above-described plasma processing apparatus, even if a standing wave is formed by having the spiral coil to resonate at ½ wavelength (or ¼ wavelength) of the high frequency, the spiral coil has only a positive or a negative voltage component without having those together at the same time. Therefore, the voltage component remains in the spiral coil all the time, and a large amount of capacitively coupled components is generated in the plasma, which makes it difficult to prevent ion damage.
In order to reduce the amount of capacitively coupled components in the plasma, the amount of voltage component remaining in the spiral coils needs to be reduced. The amount of capacitively coupled components in the plasma can be reduced by using the spiral coil having a low inductance as described in Patent Document 1. However, in case of using the spiral coil having the low inductance, the excited magnetic field becomes weak. As a result, it is difficult to generate an intense inductively coupled plasma, and a plasma density decreases.
In the Patent Document 2, a spiral coil is wound around an evacuable reaction chamber elongated in a longitudinal direction to form a standing wave by having the spiral coil to resonate in a full wavelength mode, a ½ wavelength mode or the like by applying a high frequency of a predetermined wavelength thereto and excite a plasma of a processing gas by forming an induction field in the reaction chamber. A voltage waveform is controlled by a wavelength control circuit for a phase and an anti-phase voltage to be symmetrical with respect to a point where the phase and the anti-phase voltage are switched, so that an inductively coupled plasma can be excited at a node having a zero potential at which the phase voltage is switched.
Since the antenna element is formed in a longitudinally wound spiral coil shape in the Patent Document 2, the waveform can be controlled by the wavelength control circuit so as to be symmetrical with respect to the point where the phase-voltage and the anti-phase voltage are switched. On the other hand, unlike the antenna element formed in the longitudinally wound spiral coil shape, an antenna element formed in the planar coil shape has a diameter gradually increasing from an inner end toward an outer end on the same plane. Thus, an inner line and an outer line divided by the point where the phase-voltage and the anti-phase voltage are switched have different reactances, and this makes it difficult to control the waveform to be symmetrical with respect to the point. Accordingly, the technique applied to the spiral coil described in Patent Document 2 cannot be applied to the planar coil.
In addition, along with the demand for miniaturized and multilayered semiconductor devices, it is required to simply and accurately control uniformity of plasma processing on a central portion and an edge portion of a substrate to be processed.